1. Field of the Invention
The present invention relates to a scanning apparatus employing therein an apparatus controlling a fluorescent lamp, and more particularly, to a scanning apparatus variably controlling a luminous intensity of a fluorescent lamp and a control method thereof.
2. Description of the Related Art
The cold cathode fluorescent lamp is widely utilized as a backlight source to illuminate the display panel of a liquid crystal display used as a display, such as in a portable notebook computer and the like, or as a light source constantly illuminating a manuscript in a scanning apparatus.
In order to drive a cold cathode fluorescent lamp mainly used in a scanning apparatus, it is common to generate a pulse wave through a switching device such as a transistor and to boost the generated pulse wave to a high voltage of more than 500 Vrms having a frequency equal to or above 200 KHz in a winding-type transformer applied to the cold cathode fluorescent lamp. The operation of the conventional fluorescent lamp controlling apparatus will be described with reference to FIG. 1, which shows a drive circuit of a fluorescent lamp used in a conventional scanning apparatus.
As shown in FIG. 1, according to the fluorescent lamp controlling apparatus, when the power supply switch S1 is turned on, a first transistor Q1 is activated by the voltage divided by a first resistor R1, a second resistor R2 and a third resistor R3, which form the voltage-dividing resistor. A first diode D1 and a first capacitor C1 protect the first transistor Q1 from counter electromotive force that is induced by a first inductor L1. The current output from the collector terminal of the first transistor Q1 is dropped to a predetermined voltage in a fourth resistor R4 and a fifth resistor R5 through the first inductor L1, and then the dropped voltage is applied to the respective base terminals of a second transistor Q2 and a third transistor Q3, and to the respective collector terminals of the second transistor Q2 and the third transistor Q3 through a second inductor L2 and a fourth inductor L4. A fifth inductor L5 is provided between the base terminals of the second transistor Q2 and the third transistor Q3, and hence only the single transistor begins to activate, resulting in the second transistor Q2 and the third transistor Q3 having an active state and a cut-off state that are alternatively iterated. The electromotive forces in opposite directions are alternatively generated in the second inductor L2 and the third inductor L3, respectively, and hence a secondary electromotive force of the high voltage having high frequency is generated in a third inductor L3 placed on the secondary side of transformer T1 which forms parallel-resonance with the second capacitor C2.
As described above, according to the conventional fluorescent lamp controlling apparatus, when the power supply switch S1 is turned on, a constant drive voltage is applied to the fluorescent lamp, whereas when the power supply switch S1 is turned off, the drive circuit is not operated and no drive voltage is applied to the fluorescent lamp. Consequently, the conventional fluorescent lamp controlling apparatus can not variably control the voltage applied to the fluorescent lamp and therefore, cannot adjust the luminous intensity of the fluorescent lamp for the respective operation modes of the scanning apparatus. Additionally, the conventional fluorescent lamp takes a long time to initially operate the fluorescent lamp because of the longer initial preheating time thereof, reducing the fluorescent lamp's lifetime, and causing higher power consumption.